Re: [Libevent-users] Clarify new behavior?

[Ralph Castain <rhc@xxxxxxxxxxxx>]

On Oct 23, 2010, at 9:42 AM, Nick Mathewson wrote:

> On Sat, Oct 23, 2010 at 5:14 AM, Ralph Castain <rhc@xxxxxxxxxxxx> wrote:
>> Hi folks
>> 
>> I successfully updated our libevent integration in Open MPI, but have encountered a problem with one use-case that used to work and now doesn't. Before proceeding to devise a fix, I just wanted to confirm that I accurately understand the issue.
>> 
>> The problem arises from this scenario:
>> 
>> 1. we receive a command via a message that we receive in a file descriptor event. We "push" the command message into a timer event (duration zero time) to help break a threading issue, and then return from the file descriptor event.
>> 
>> 2. the event library is called with LOOP_ONCE, causing the timer event to fire.
>> 
>> 3. from within the timer event, the command causes us to execute a procedure that results in us having to wait for another event to occur. We "block" in that position, running a loop that includes a call to progress the event library (i.e., a call to event_loop(LOOP_ONCE)).
>> 
>> This last step used to work just fine, yet now yields the following warning:
>> 
>> warn] event_base_loop: reentrant invocation.  Only one event_base_loop can run on each event_base at once.: Resource temporarily unavailable
>> 
>> and no progression of the event library occurs.
>> 
>> I can see a few ways around this problem, but all involve considerable code rewrite. Before I do so, I just wanted to confirm my understanding that this behavior changed (i.e., recursion is no longer allowed), and see if any alternative approaches to revising such codes are recommended.
> 
> Hi, Ralph!  I'll try to answer in more detail later on -- I can't
> write too much now, but I wanted to drop you a line before you started
> rewriting your code.
> 
> I think that the warning you hit may be overzealous.  It was meant to
> detect errors that would previously pass silently and produce
> hard-to-diagnose crashes, such as running event_base_dispatch on the
> same event base from multiple threads at once.  But I think your use
> case may either a) work right, or b) be trivial to make work right,
> and if so, we should fix Libevent not to warn about it.
> 
> I'll look harder at the code here and try to see what's going on here,
> but calling the loop from a callback really was working fine with
> Libevent 1.4, then I think 2.0 should support it.

FWIW: I decided to just try commenting out the "return" following the warning in event.c, and everything works fine.

Not recommending that as a permanent fix, of course - it may just work due to our peculiar usage. After all, we are running single-threaded, and so you can't call the dispatch from multiple threads. But thought you might like to know.

> 
> Thanks for the heads-up, and thanks to everybody else who's hunting
> for regressions and other bugs in 2.0.8-rc.
> 
> yrs,
> -- 
> Nick
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